The present invention relates to a device and method for referencing reference points of a fiducial implant.
In Computer Assisted Surgery (xe2x80x9cCASxe2x80x9d), a computer tomogram comprising an image of a portion of a body, such as a bone, is generally used to assist the surgical intervention. For example, to determine the relative positions of a bone and one or more surgical instruments, the positions of the bone and surgical instrument in a coordinate system of the operating room can be compared with their positions as displayed in a computer tomogram.
To reduce soft tissue injury, minimally invasive surgical techniques involving CAS can be used to operate on bones, such as, for example, vertebrae, without exposing large bone surfaces. In CAS, a coordinate system corresponding to a patient in the operating room, as determined by a dynamic reference base of a position finding system, is registered with a coordinate system of a previously recorded tomographic image. This process, called matching involves a transformation between the coordinate systems.
A more comprehensive description of computer assisted surgery including a matching algorithm suitable for such coordinate transformations is given in L. P. Nolte et al, Clinical Evaluation of a System for Precision Enhancement in Spine Surgery, Clinical Biomechanics, 1995, Vol. 10, No. 6, PP 293-303, which is hereby incorporated by reference in its entirety.
One way to match the coordinate system of the patient in the operating room to the coordinate system of the image is to use mechanical scanning devices, such as those described in U.S. Pat. No. 5,383,454 to Buchholz. However, these known procedures are exceedingly time-consuming and presently only of obsolete accuracy.
German Application No. DE 29 704 393 to AESCULAP discloses a process for the preoperative determination of positioning data of endoprosthesis parts of a central joint relative to the bones that form the central joint. A device for determining the outer articulation point of a joint as the bones are displaced is also disclosed.
Another way of matching the patient""s system of coordinates in the operating room with the image coordinate system consists of identifying predetermined points using anatomical reference points. Where the body parts are only partly exposed, however, this method is complicated by the restricted visual access. Frequently an endoscope must be used.
In another method to register the systems of coordinates, so-called fiducial implants are used to unequivocally identify the reference points. Such a method and appropriate apparatus are described in U.S. Pat. No. 4,945,914 to Allen. This method includes the implantation of at least 4 spatially related fiducial implants.
All these known methods share the drawback of being fairly time-consuming.
The present invention relates to a device for referencing reference points of a fiducial implant. The device preferably comprises at least one fiducial implant, such as a fiducial screw, suitable for insertion into a bone. The fiducial implant includes at least first and second reference points that have a known spatial relationship with respect to a head of the implant. The device also includes a pointer having a distal pointer end and at least three emitters or detectors that have a known spatial relationship with respect to the distal pointer end. Preferably, the emitters or detectors are arranged in a plane perpendicular to the longitudinal axis of the pointer. Each emitter or detector is configured to emit or detect radiation. The radiation preferably comprises electromagnetic radiation or acoustic waves. Preferably, the pointer includes emitters of electromagnetic radiation, such as light emitting diodes.
In one embodiment of the present invention, the pointer includes emitters and the device further includes a position finder comprising at least two detectors configured to detect the radiation emitted by the emitters of the pointer, whereby the positions of the emitters of the pointer and the pointer in three-dimensional space can be determined. Preferably, the detectors detect electromagnetic radiation or acoustic waves.
In another embodiment of the present invention, the pointer includes detectors and the device further includes a position finder comprising at least two emitters configured to emit radiation able to be detected by the detectors of the pointer, whereby the position of the detectors of the pointer and the pointer in three-dimensional space can be determined. Preferably, the emitters emit electromagnetic radiation or acoustic waves.
The head of the implant and the distal pointer end are preferably configured to removably and reproducibly mate to provide a known spatial relationship between the reference points and the distal pointer end to allow the position of each reference point in three dimensional space to be determined upon determining the position of the emitters or detectors of the pointer. The device of the present invention preferably includes a processor configured to perform a coordinate transformation to match the position of each reference point in three-dimensional space with a corresponding position of the respective reference point in a stored image of the bone.
In a preferred embodiment, the device comprises a second fiducial implant able to be inserted in a bone at a preferably spaced apart position from the first fiducial implant. The second fiducial implant is preferably configured with a head and reference points identical to the distal fiducial implant. The head of the second implant and the distal pointer end can preferably removably and reproducibly mate to provide a known spatial relationship between the reference points of the second fiducial and the distal pointer end to allow the positions of the reference points in three dimensional space to be determined.
In the preferred embodiment, the processor is configured to match the positions in three-dimensional space of the reference points of the second fiducial with the corresponding positions of the respective reference points in the stored image of the bone. The processor is also configured to perform a coordinate transformation between coordinates of points in three-dimensional space with the corresponding coordinates of the points in the stored image. Thus, the position of a selected point of the bone can be matched with the corresponding position of the selected point in the image of the bone and a selected point of an image of the bone can be matched with the corresponding point of the bone in physical space.
The head of the implant of the invention preferably comprises a cavity and the first end of the pointer preferably comprises a pin configured to be received within the cavity to provide the known spatial relationship between the reference points of the implant and the distal end of the pointer by means of the known dimensions of the implant and pointer and the reproducible mating between the implant and the pointer. Preferably, the distal end of the pointer comprises a shoulder adjoining the pin with the diameter of the shoulder being larger than the diameter of the pin. The shoulder is configured to abut the head of the implant when the pin is received within the cavity. Preferably, the pointer can be used to screw in or screw out the fiducial screw when the pin is received within the cavity.
Another embodiment of the invention comprises a method for creating a spatial relationship between reference points associated with a body. The method comprises inserting at least two fiducial implants in the body, each fiducial implant comprising at least two reference points. An image of the body showing at least two of the inserted fiducial implants is preferably obtained. The position in three-dimensional space of the reference points of at least two of the fiducial implants shown in the image is determined. A coordinate transformation is preferably performed to match the positions of the reference points in three-dimensional space with corresponding positions of the reference points in the image.
Preferably the step of determining the position of the reference points in three-dimensional space comprises mating a distal end of a pointer to a head of the implant and determining the position of the pointer using a position finder. Upon determining the position of the pointer, the position of the reference points can be found by means of the known dimensions of the implant and pointer and the reproducible mating between the implant and the pointer. The image is preferably a three-dimensional computer tomogram or an x-ray.